And both he and the Others are within a larger world of Aliens who are less organized and less kind, and this is possibly explained by them being less smart. Not the ones who built the isekai machinery, as an obvious guess, but perhaps a fallen version of a civilization that did?
He knows next to nothing about them, and maybe that's because he didn't ask the right questions, but it still feels like there's some kind of gap of -- it being assumed he'd guess more, or know more, about the Aliens, than he does. It is very noticeable how little Othrem has ended up knowing about the Aliens despite trying to ask for concrete examples about them.
And from her story, it sounded like -- the Aliens haven't noticed the Others? Or the Others pass among them without it being an explicit First Contact situation with diplomacy? If the Others pass among the Aliens unnoticed, and if all correlation is assumed to be due to shared ancestry, then the Aliens are also cousins of dath ilan but perhaps far more distant ones. Sharing a humanoid shape, maybe, but who knows how much or how little else.
...Othrem would absolutely not assume he understood an Alien civilization well enough to do guerilla-optimization to their street signs. He'd be playing a much more paranoid and cautious game to avoid being noticed, if it had been decided to maintain the unawareness of the Aliens of the Others in their midst. But maybe there was some kind of misleading sense of similarity that misled that Other into thinking they knew a helpful and nondisruptive course of action. Or maybe it's an extension of the Others' increased affection -- something like, increased trust, increased jumping right into things and trying to help. Othrem thinks that even if he felt an emotion like that, it would be easy for him to notice and override when a larger goal weighed against that; but maybe that's an illusion-of-imaginary-competence, born of him not feeling that strongly in the first place.